Diagonal of a Parallelogram Divides it into Two Congruent Triangles


 
 
Concept Explanation
 

Diagonal of a Parallelogram Divides it into Two Congruent Triangles

Diagonals of a Parallelogram: We know that parallelogram is a quadrilateral in which both the opposite pair of sides are parallel and equal to each other. The diagonal of a parallelogram divides it into two congruent triangles.

Theorem 1: A diagonal of a parallelogram divides it into two congruent triangles.

Given  A parallelogram ABCD.

To prove  A diagonal, say, AC, of parallelogram ABCD divides it into congruent triangles ABC and CDA i.e. large Delta ABC large cong large Delta CDA

Construction  Join AC.

Proof  Since ABCD is a parallelogram. Therefore,   AB large parallel DC and AD large parallel BC

Now, AD large parallel BC and transversal AC intersects them at A and C respectively.

large therefore   <DAC = <BCA    ....(i)          [ Alternate interior angles ]

Again, AB large parallel DC and transversal AC intersects them at A and C respectively. Therefore,

      <BAC = <DCA    ....(ii)          [ Alternate interior angles ]

Now, in large Deltas ABC and CDA, we have

       <BCA = <DAC                    [From (i)]

           AC = AC                      [ Common side ]

       <BAC = <DCA                    [ From (i)]

So, by ASA congruence criterion, we have

     large Delta ABC large cong large DeltaCDA

ILLUSTRATION: Find the value of x and y in the figure

Solution:  In the figure ABCD is a parallelogram and AC is the diagonal. We know that the diagonal of a parallelogram divides it into two congruent triangles

   Delta ABC cong Delta CDA

  angle BAC = angle DCA         [ Corresponding parts of congruent triangle]

   Rightarrow x= 55^0

Similarly    angle BCA = angle DAC         [ Corresponding parts of congruent triangle]

   Rightarrow y= 30^0

Sample Questions
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Question : 1

In the following figure, ABCD is a parallelogram. The diagonal AC divides this parallelogram into which congruent triangles?

Right Option : B
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Explanation
Question : 2

In the following figure, Diagonal AC divides the inner parallelogram into which two congruent triangles?

Right Option : C
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Explanation
Question : 3

In the following figure, diagonal AD divides parallelogram into which two congruent triangles?

Right Option : B
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Explanation
 
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